Apparatus for preparing superconducting joints

ABSTRACT

An apparatus for forming a superconductive joint in tape or wire has features which include a closeable vessel with gas inlets for depositing a superconducting layer by chemical vapor deposition, and a form in combination with current clamps and conductive leads to heat a length of tape to a temperature effective to deposit a superconductive coating thereon. Preferably, the form and associated structure are unitary with a base plate which seals the vessel.

This invention relates to superconductors, and more particularly to thepreparation of superconducting joints.

Superconducting coils are widely used or have potential use in variousapplications such as in accelerators and nuclear magnetic resonanceimaging equipment. In general, these coils are formed by windingsuperconducting wire or tape around a core element. The word "tape" asused hereinafter includes both tape and wire, although in its primarymeaning it designates an elongate body having relatively large lengthand width dimensions and a small thickness dimension.

In general, superconducting tapes are prepared by depositing asuperconducting layer on a core of a parent metal such as niobium,tantalum, titanium or vanadium, most often containing small percentagesof other materials such as zirconium and oxygen. The core is necessaryto afford strength to the tape, since the superconductive materialtherein is typically an intermetallic material with essentially nointrinsic strength. It most often comprises a combination of the parentmetal with a reactive metal or metalloid such as tin, aluminum, silicon,gallium or germanium. Tin is most often employed by reason of itsavailability, relatively low cost and suitability. A superconductivematerial in common use corresponds generally to the formula Nb₃ Sn,sometimes designated hereinafter "triniobium tin".

A superconducting layer of triniobium tin on a niobium core may beprepared by passing said core continuously through a bath of tin or atin alloy, whereupon the core picks up a thin layer of tin which, uponheating, forms triniobium tin in the region nearest the core. Theconnective and superconductive layer thus obtained becomes enriched intin as the distance from the core increases with the outer surface beingessentially pure tin. It is also known to employ for this purpose achemical vapor deposition (hereinafter sometimes "CVD") process in whicha combination of tin and niobium halides is reduced by elementalhydrogen to the elemental metals, which deposit on the core.

Further steps are generally necessary to produce a tape of the desiredstrength and ductility. Most often, a cladding layer of a substantiallychemically inert but electrically conductive metal (e.g., copper) isapplied on the outer surface of the superconducting layer to protect itfrom chemical attack and to provide an electrical shunt path.Application of this cladding layer is typically by soft soldering whensaid outer surface is essentially tin. In certain instances, aninsulator such as varnish may in turn be applied to the cladding layer.Similar methods may be employed for the production of superconductingtapes containing other core and superconductive materials.

Present and future generations of superconducting devices require verylong superconducting coils, often substantially greater than onekilometer. However, present technology makes it difficult to fabricatecontinuous superconducting coils with lengths greater than about 300meters. For the preparation of longer tape coils, therefore, it is oftendesirable to splice together several lengths of tape, with the jointsbetween such lengths themselves being superconducting.

The preparation of superconducting joints between successive lengths oftape presents several daunting obstacles. In order for operation in apersistent superconductive mode to be practical, there should be nosubstantial current loss in passage through the joint; otherwise,continual replenishment of power will be necessary, substantiallyincreasing the cost of the apparatus. In addition, the joint isfrequently a heat source by reason of its relatively high resistivity,and may propagate a quench front of an intensity too great for handlingby the cladding layer. If that happens, superconductivity in the entirecoil will be quenched. One result might be a release of catastrophicallyunsafe amounts of magnetic energy.

Various methods for producing superconducting joints are known in theart. One such method, disclosed in U.S. Pat. No. 4,744,506, involves theuse of a superconductive lead-bismuth solder. Such solders are not,however, operative in very high magnetic fields. Moreover, they passonly a relatively low current density, which means that solder joints onthe order of one meter or greater in length may be necessary in asuperconducting coil.

More recently, various methods of producing superconducting joints bywelding have been made available. Copending, commonly owned U.S. Pat.No. 5,109,593 discloses a method in which the substantially inert metallayer is removed from tape ends to be joined, after which the exposedinner sections typically comprising niobium and triniobium tin areplaced in contact, melted and resolidified. A continuous precipitate ofthe superconductive alloy is thus formed.

Another welding method is disclosed in copending, commonly owned U.S.Pat. No. 5,082,164. In this method, the substantially inert metal layerand the superconducting layer are both removed from the tape ends to bejoined, exposing the core. The core sections are then placed in contactand heated in a protective atmosphere such as helium or argon, in thepresence of excess reactive metal which forms a continuous layer of thesuperconductive alloy.

These welding methods are, in many ways, superior to previously knownmethods for joining superconducting tapes. However, there is still asubstantial probability of failure of at least one welded joint in alength of tape containing many such joints. It would be desirable,therefore, to provide an alternative method for preparingsuperconducting joints which could be used either in place of welding,or in addition to welding to provide a backup superconductive path.

In particular, CVD method for producing superconducting joints would beadvantageous. Since CVD methods for producing continuous lengths ofsuperconducting tape are known, it would at first sight appear to be asimple matter to adapt one of them to the fabrication of joints.However, the tape CVD methods are based on steady state operation, whichis not feasible for the fabrication of numerous isolated joints.

Thus, there still exists a need to develop a CVD method for makingconnective superconducting joints, as well as apparatus in which saidmethod can be conducted. The present invention provides such anapparatus.

Accordingly, the invention includes apparatus for forming asuperconductive joint, said apparatus comprising:

a closable vessel resistant to metal halides in the vapor state,comprising a housing and a top plate;

feed means in said vessel for reactive and inert gases;

exhaust means in said vessel for venting by-product gases;

base means adapted for sealable mounting to form the closure of saidvessel;

entry slots in said base means to accommodate an internal length of anelongated superconductor, said entry slots being adapted to sealablycontact said superconductor when said base means is mounted in saidvessel;

forming means for forming a superconductive joint in saidsuperconductor, said means comprising a heat-resistant and chemicallyinert curved mount for said superconductor with a gap in the surfacethereof;

heat-resistant securing means for securing said superconductor to saidforming means; and

current passage means for passing an electric current through saidsuperconductor when in contact with said forming means.

In the drawings, which represent views of a CVD apparatus according tothe invention:

FIGS. 1 and 2 are transverse views of opposite ends of the apparatus,the former being shown partially disassembled;

FIG. 3 is a view corresponding to that of FIG. 1, but orthogonallydirected and showing the apparatus assembled; and

FIG. 4 is an axial view of the same apparatus along the line 3--3 ofFIG. 1.

The apparatus of this invention is particularly suitable for use with amethod for producing a superconducting joint between ends of continuoussuperconductors, each of said ends comprising a parent metal core and asuperconductive alloy layer thereon, said superconductive alloy layercomprising a combination of said parent metal and at least one reactivemetal; said method comprising the steps of:

(A) removing any non-superconductive material from said ends;

(B) placing said ends in contact and physically joining them with anelectrically conductive joint;

(C) depositing a connecting superconductive alloy layer on said ends bya chemical vapor deposition reaction of hydrogen with halides of saidparent and reactive metals at a temperature in the range of about700°-950° C.;

(D) cooling said joined ends;

(E) depositing on said joined ends a solder-accepting pure metal; and

(F) covering said joined ends with a substantially chemically inert butelectrically conductive cladding layer;

step C being conducted in an anhydrous inert atmosphere free fromelemental oxygen, and step D in an anhydrous inert atmosphere free fromelemental hydrogen and elemental oxygen. Said method is disclosed andclaimed in copending application Ser. No. 07/802,970.

The parent metals and reactive metals employed in the superconductingtapes treated by the above-summarized method are known in the art, andmany of them are listed hereinabove. In general, the preferred parentand reactive metals are niobium and tin, respectively. It mayoccasionally be advantageous to substitute germanium for tin as thereactive metal.

As previously mentioned, superconducting tapes generally have a claddinglayer of a substantially inert metal such as copper over a layer ofsubstantially pure tin. Both of these layers are essentiallynon-superconducting, or at least do not pass the required currentdensity levels superconductively. The substantially pure tin forms theouter portion of the triniobium tin superconducting layer. It isnecessary in step A to remove any such non-superconductive materials.This may be achieved by etching the tape ends, typically to a length ofabout 4-5 cm., with an acid which is reactive with thenon-superconductive metal(s). Concentrated mineral acids areparticularly useful for this purpose, with nitric acid generally beingpreferred when the cladding metal is copper.

After removal of the non-superconductive material, in step B the tapeends are physically joined with an electrically conductive joint. Thisis typically achieved by a welding operation. The joint thus obtainedneed not be superconductive, but of course if a backup superconductivepath is desired, a superconducting joint must be provided. One of thewelding operations disclosed in the aforementioned commonly ownedapplications, the disclosures of which are incorporated by referenceherein, may be employed for this purpose. Particularly preferred, byreason of its simplicity, is the method of Ser. No. 07/561,438 whichrequires only a single welding step without multiple strippingoperations.

Steps C and D must be conducted in an anhydrous inert atmosphere.Because of the reactivity of niobium, in particular, with both elementaloxygen and elemental hydrogen, it is necessary for said atmosphere to befree from elemental oxygen, and in step D also from elemental hydrogen.Inert gases such as argon and helium may be employed for this purpose.Argon is, however, usually obtained from air and may contain minorproportions of oxygen. Therefore, helium is the preferred inertatmosphere material.

In step C, a connecting superconductive alloy layer is deposited on saidtape ends. This is accomplished by a CVD reaction between hydrogen andhalides of the parent and reactive metals. Any halides of said metalsmay be employed, in substantially pure form or as constituents of amixture. The preferred halides are the chlorides, of which examples aretin tetrachloride, niobium pentachloride and germanium tetrachloride.They may be employed as such and/or may be prepared by passage ofchlorine over a body of elemental metal, most often niobium orgermanium.

The CVD reaction takes place at a temperature in the range of about700°-950° C., preferably about 800°-900° C., and typically atatmospheric or near-atmospheric pressure. It is typically achieved byinterrupting the flow of inert gas and replacing it with a flow of themetal halides and hydrogen, or by directing a mixed flow of inert gasand chlorine over at least one elemental metal and combining theresulting mixture of helium and metal chloride with hydrogen and anyother metal chloride employed.

To avoid premature reaction between the hydrogen and metal halide(s), itmay be advisable, particularly if they are introduced at elevatedtemperatures, to initiate contact between them only at the time of entryinto the vessel in which the tape ends are contained. They may beintroduced through separate ports.

Pressures in the reaction vessel during the CVD reaction are typicallyatmospheric or, preferably, slightly above atmospheric to suppress entryby oxygen- and moisture-containing gases. Ratios of gases--inert gas toreactive gases and hydrogen to metal halide--do not appear to becritical provided a sufficient amount of metal halide is employed todeposit the required amount of metal.

Molar ratios of the metal halides used in the CVD reaction are notusually stoichiometric (e.g., 3 moles of Nb to 1 mole Sn) but varyaccording to the reaction conditions. Ratios of the order of about 1:4(Nb:Sn) are typical when chlorides of both metals are employed as such.When niobium chloride is prepared by passage of chlorine over elementalniobium in certain laboratory equipment, it may be necessary to employan excess of chloride with respect to said ratio. It is believed thatthe higher proportion of chlorine required in the latter instance is theresult of a portion of the chlorine not coming into contact with themetallic niobium. In any event, the precise proportions to be used maybe determined by simple experimentation.

Step D is the cooling of the joined tape ends, most often to atemperature below about 40° C. This must also be performed in an inertanhydrous atmosphere free from elemental hydrogen and elemental oxygen,typically with the employment of inert gas identical or similar to thatemployed during step C. The inert atmosphere is necessary at this stageto aboid the formation of friable oxides or hydrides of parent metal,especially niobium, which are often preferentially formed if any oxygen,hydrogen or moisture contacts the superconducting layer. In general, theflow of metal chlorides and hydrogen is immediately replaced by a flowof inert gas such as helium before the cooling step begins, and saidflow is maintained as the tape ends are cooled.

Step E is the deposition on the joined tape ends, having depositedthereon the superconducting layer, of a pure metal layer which willaccept solder. A metal layer is necessary because of the lack ofavailability of solders which wet niobium or alloys thereof. It must bepure by reason of the required high electrical conductivity of thesolder-accepting layer, which must be effective to suppress theformation of "hot spots" which might quench superconductivity.

If any oxide of the superconductive material(s) has been formed, it mustbe removed before the pure metal layer is applied. This is typicallyachieved by contacting the joined tape ends with an aqueous hydrofluoricacid solution, typically about 40-60% HF by weight, preferably byimmersion.

Suitable pure metals include nickel, gold and platinum, which arenormally deposited by conventional electrolytic means. Electrolessdeposition is generally not acceptable since it often involves the useof other elements in the form of phosphorus compounds, for example. Thepreferred metal under most circumstances is nickel, by reason of itsrelatively low cost and particular suitability.

The superconducting joints may be tested for superconductivity followingthe completion of step E. Before use, however, it is necessary to coverthe joined tape ends with a cladding layer, which constitutes step F ofthe method. This is typically achieved by applying to the pure metallayer a conventional solder, typically a tin-lead solder, followed by acopper cladding material similar or identical to that present onsuperconducting tape.

The product obtained by the above-described method is a superconductingjoint between multiple lengths of superconducting tape. Only two suchlengths are usually involved, although it is within the scope of theinvention to produce a branching joint from three or even more lengths.

The drawings illustrate an apparatus according to the present inventionin which step C of the above-described method may be conducted. Saidapparatus, generally designated 10, includes a vessel in which the CVDreaction is performed, said vessel including housing 1 and top plate 8.Said vessel is constructed of suitable material resistant to metalhalides in the vapor state, with glass being particularly advantageousfor most of housing 1 and substantially chemically inert metal for topplate 8.

Gases are fed to the vessel through inlets 5 and 9, shown in FIG. 2:reactant gases through inlet tube 9 which terminates in proximity to theform described hereinafter (as shown in FIGS. 1 and 4), and inert purgegas through inlet 5, passing via feed tube 7 to near the bottom of saidvessel (as shown in FIG. 4) to facilitate rapid and efficientdisplacement of reactant gases by inert gas when the CVD operation hasbeen completed. The gaseous contents are removed from vessel 1 throughvent 3. The length of vessel 1 is typically about 25-50 cm.

Base plate 2 is sealably fastened to housing 1 by bolts 4, the sealbeing provided by rubber O-ring 23. As shown in FIG. 3, entry slots 19for superconducting tape are cut into the periphery of base plate 2;each of said tape entry slots is lined with a seal (not shown) of softresilient material, typically silicone rubber, to prevent breakage ofthe tape.

Form 11, cylindrical in shape and typically about 4-6 cm. in diameter,is fabricated of a suitable chemically inert, electrically insulatingand heat-resistant material such as alumina, silicon nitride, siliconcarbide or hexagonal boron nitride, the latter often being preferred. Itis fitted with gap 13 and groove 14, said gap having a circumferentialwidth of about 1-2 cm. and said groove being cut in the circumference ofthe form in the area of contact with the superconducting tape and havinga width to accomodate said tape.

Current clamps 29, typically constructed of a relatively soft,heat-resistant and electrically conductive material such as graphite,are electrically powered via rigid leads 33 which are fastened to faceplate 2 and which rigidly pass through form 11 so that the face plate,leads, current clamps and form are of unitary construction. Said leads33 are insulated from the face plate by insulators 34 and fastenedthereto by fittings 35. Said current clamps are attached to form 11 bybolts 16 (only one being shown in FIG. 4), in such a way that they canbe securely fastened so as to prevent movement of superconducting tapewhen mounted in the apparatus as explained hereinafter.

Following the physical joining of two lengths of superconducting tapeaccording to step B of the above-described method, apparatus 10 isdisassembled by removal of face plate 2 and attached parts fromhousing 1. Current clamps 29 are loosened and an internal length (i.e.,a length not including an end) of tape 15, including the joint, isfirmly seated in groove 14 of form 11, with the joint between the twotape portions bridging gap 13. Current clamps 29 are then tightened soas to secure the tape in place, and face plate 2 is fastened to housing1 by means of bolts 4. A substantially hermetic seal is provided uponclosure, by O-ring 23 and the seals on entry ports 19.

Upon passage of an electric current through said clamps, the tape jointwhich is exposed via gap 13 are heated to a temperature at which the CVDreaction takes place. Prior to current passage, an inert gas such ashelium is supplied to the vessel via inlet 5; it is replaced (at leastin part) by reactive metal halide and hydrogen gases introduced viainlet 9. The pressure in the vessel is at least atmospheric, andpreferably slightly greater than atmospheric to prevent entry ofatmospheric gases.

Upon heating the exposed region of the tape ends to a temperature in therange of about 700°-950° C., chemical vapor deposition of thesuperconductive alloy layer thereon takes place. When a layer ofsuitable thickness has been formed, the flow of reactive gases isdiscontinued and the apparatus charged again with inert gas and cooled.Bolts 4 are removed, housing 1 and face plate 2 are separated and bolts16 securing current clamps 29 are also loosened so that the joined tapecan be removed for further processing. Another tape joint can then besecured in the apparatus and the process repeated.

It will be apparent to those skilled in the art that the invention isnot limited by the above description of illustrative means forperforming the various functions of the apparatus. For example, othertermination points for the gas feed means are possible or the form maybe elliptical or may have some other curved form rather than beingcylindrical. Other possible variations will be apparent to those skilledin the art.

What is claimed is:
 1. Apparatus for forming a superconductive joint,said apparatus comprising:a closable vessel resistant to metal halidesin the vapor state, comprising a housing and a top plate; feed means insaid vessel for reactive and inert gases; exhaust means in said vesselfor venting by-product gases; base means adapted for sealable mountingto form the closure of said vessel; entry slots in said base means toaccommodate an internal length of an elongated superconductor, saidentry slots being adapted to sealably contact said superconductor whensaid base means is mounted in said vessel; forming means for forming asuperconductive joint in said superconductor, said forming meanscomprising a chemically inert, electrically insulating andheat-resistant curved mount for said superconductor with a gap in thesurface thereof; heat-resistant securing means for securing saidsuperconductor to said forming means; and current passage means forpassing an electric current through said superconductor when in contactwith said forming means.
 2. Apparatus according to claim 1 wherein saidreactive gas feed means includes an inlet tube terminating in proximityto said forming means.
 3. Apparatus according to claim 1 wherein saidinert gas feed means includes a feed tube terminating near the bottom ofsaid vessel.
 4. Apparatus according to claim 1 wherein said formingmeans is cylindrical in shape.
 5. Apparatus according to claim 4 whereinsaid forming means has a groove in the circumference thereof in the areaof contact with the superconductor and of a width to accommodate saidsuperconductor.
 6. Apparatus according to claim 4 wherein said formingmeans is fabricated of hexagonal boron nitride.
 7. Apparatus accordingto claim 4 wherein said securing means are fabricated of graphite. 8.Apparatus according to claim 1 wherein said base means, forming means,securing means and current passage means are of unitary constructionwith said current passage means being electrically insulated from saidbase means.
 9. Apparatus for forming a superconductive joint, saidapparatus comprising:a closable vessel resistant to metal halides in thevapor state, comprising a housing and a top plate; a feed tube in saidvessel for reactive gases; a feed tube in said vessel for inert gases,terminating near the bottom of said vessel; exhaust means in said vesselfor venting by-product gases; a base plate adapted for sealable mountingto form the closure of said vessel; entry slots in said base plate toaccommodate an internal length of superconducting tape, said entry slotsbeing lined with a seal of soft resilient material adapted to sealablycontact said superconducting tape when said base means is mounted insaid vessel; a cylindrical form for forming a superconductive joint insaid superconducting tape, said form comprising a chemically inert,electrically insulating and heat-resistant curved mount for saidsuperconductor with a gap in the surface thereof and a groove in thecircumference thereof in the area of contact with said superconductingtape and having a width to accommodate said tape; said feed tube forreactive gases terminating in proximity to said form; a pair of currentclamps for securing said superconducting tape to said form; and rigidcurrent leads for passing an electric current through saidsuperconductor when in contact with said forming means; said base plate,form, current clamps and current leads being of unitary constructionwith said current leads being electrically insulated from said baseplate.
 10. Apparatus according to claim 9 wherein said form isfabricated of hexagonal boron nitride.
 11. Apparatus according to claim9 wherein said current clamps are fabricated of graphite.